In order to be able to handle a load in a rational manner, a truck often requires a load handling unit. The most common examples of such load handling equipment are tippers and cranes. Demountable load carriers such as a hook-lift, refuse handling equipment, rotating cement mixers and compressors for loading and unloading bulk loads are the most common. In order to utilise the engine of the truck to provide power for this load handling equipment it is necessary to fit a power take-off unit, often referred to as a PTO. Power from this unit can be transferred either mechanically via a propeller shaft or belts, or hydraulically by fitting a hydraulic pump to the power take-off. Power take-offs are divided into two categories, those independent of a clutch in the vehicle transmission and those dependent on this clutch.
A clutch-independent power take-off or PTO may be fitted to the engine or between the engine and the gearbox. A PTO of this type may be operated whenever the engine is running. The speed and power output of the PTO is directly dependent on the engine speed, irrespective of whether the truck is being driven or is at a standstill. Clutch-independent PTOs are suitable for devices such as load handling equipment, which is used when the truck is being driven. Examples of load handling equipment are demountable mechanical and hydraulic units, refuse trucks, cement mixers and snow ploughs.
A clutch-dependent power take-off is fitted to a gearbox or a transmission adjacent the gearbox and may be driven by, for example, an idler shaft in the gearbox. This means that the PTO can only deliver hydraulic or mechanical power when a clutch between the engine and the gearbox is engaged. The PTO speed is dependent on the engine speed and the speed of the idler shaft. If the gearbox has a range splitter, the range selection may affect the speed of the idler shaft and thereby the speed of the PTO. The PTO can be connected to a hydraulic pump, for delivering hydraulic pressure, or to a propeller shaft, for delivering mechanical power or torque. Some PTOs may feature double connection to provide both a hydraulic pump and/or a mechanical propeller shaft.
In a hydraulic system, a hydraulic pump, which delivers hydraulic pressure and flow to an auxiliary equipment to be driven, is connected to the power take-off. There are different types of hydraulic PTOs. One type of PTO is fixedly connected to the engine or between the engine and the gearbox, as described above. This type of PTO is constantly engaged and is suitable for the connection of a PTO driving a hydraulic pump. The driven auxiliary equipment can be started and stopped using a valve unit provided on the PTO, on the auxiliary equipment or at a suitable position on the vehicle and controls the flow of hydraulic oil through the auxiliary equipment. In this case the hydraulic pump is constantly delivering pressurized oil, either to the equipment or to an oil reservoir. This type of PTO is suitable for vehicles that regularly use driven auxiliary equipment.
In order to allow a hydraulic pump in a PTO to idle, a bypass valve may be provided. The bypass valve is controlled by the user and allows the oil to bypass the hydraulic pump when the equipment is not used. This is done by actuating the bypass valve to connect an outlet port of the hydraulic pump to an inlet port thereof. The pump will then be running idle with a relatively low resistance, which may reduce the losses caused by driving the pump unnecessarily and may also minimise the wear of the pump. This type of PTO is also suitable for equipment that is used regularly and is suitable for use when the vehicle is running.
Other types of PTO are possible to engage and disengage from the engine or the transmission. One such PTO uses a claw coupling wherein a coupling socket couples together two splined shafts by means of a shift fork. This type of PTO allows disengagement during use. However, in order to engage the PTO both splined shafts must be at a complete standstill. Consequently, when this type of PTO is mounted on a gearbox, the vehicle must stand still with the clutch disengaged when the shift fork is actuated. This type of PTO is therefore not always suitable for mounting on the engine or between the engine and the gearbox.
A further type of PTO uses a hydraulically actuated multi-disc clutch to engage and disengage the PTO. This arrangement makes it possible to engage and disengage the PTO when the input shaft is rotating. Hence, this type of PTO may be mounted both on the engine and on the gearbox, and can be used for driving auxiliary equipment that needs to be engaged when the vehicle is being driven.
A problem relating to a PTO in direct connection with the engine or the gearbox is that the hydraulic system can not be disengaged. As described above, the pump is circulating the fluid of the hydraulic system whenever the engine is running. Consequently the pump circulates the fluid through the system even if the auxiliary equipment is not in use. This is a problem, as the fuel consumption of the vehicle is increased unnecessarily when the auxiliary equipment is not used. Furthermore, when the auxiliary equipment is inactive, this circulating of fluid in the tubes of the hydraulic system by the pump causes unnecessary wear of the hydraulic system. Subsequent repairs or replacement of worn out parts induces further cost.
An additional problem with a PTO that can not be disengaged is that the vehicle can not be driven if there is a leak in the hydraulic system, e.g. if a hydraulic line is ruptured or otherwise damaged. As the PTO is constantly engaged, operation of the vehicle will cause the hydraulic fluid to leak out. Apart from causing a spill of hydraulic fluid, lack of fluid in the hydraulic system may cause the hydraulic pump to break down due to lack of lubrication. To avoid this, the hydraulic system must either be repaired in situ, or the vehicle must be towed to a repair facility.
In order to solve the problems relating to increased fuel consumption and unnecessary wear of components in the hydraulic system, known systems have introduced a bypass valve. The bypass valve disconnects the hydraulic system from the hydraulic pump, so that the pump is idling. This may give a reduction of the wear in the hydraulic system and reduces fuel consumption. However, the PTO is still engaged and the pump is still running. A problem with this type of PTO is that the hydraulic system is driven directly by the gear transmission of the engine when the vehicle is being driven. Consequently, the vehicle can not be driven without causing a spill of hydraulic fluid and may cause the hydraulic pump to break down, as stated above.
A solution to the problem of having a PTO with an idling hydraulic pump is to use a clutch that allows for a complete disengagement of the PTO from the engine or gearbox, as described above. Although this system does allow disengagement of the PTO, it is a relatively heavy and costly system. A further problem is that a standard interface for such a PTO has to be ordered and installed during production of a stationary engine or a vehicle engine. The standard interface has an input side connected to a suitable drive shaft or similar on the engine or the gearbox. The standard interface allows a mechanical drive unit or hydraulic pump to be connected for driving auxiliary equipment. Post-production fitting, or retrofitting, a standard interface for such a system is expensive and may, providing that it is at all possible, require substantial modification of the engine and/or the gearbox.
Although original vehicle manufacturers have supplied general purpose hydraulic pumps with vehicles suitable for supporting hydraulic power take off operation, the provision of controls and hydraulic lines has generally been left to after market specialists. Such controls have been retrofitted to vehicles, typically by the addition of wiring and hydraulic lines. Integration with other vehicle systems has been poor and the location of hydraulic tanks and modification of vehicle body systems can compromise owners' chassis warranties. Nor have these accessories been as reliable, light weight, or small as possible, nor have they conveniently provided as much power as can be required without modification of a vehicle's hardware.
It is accordingly an object of the present invention to provide a power generating unit that overcomes the above problems. Such a power generating unit aims to reduce fuel consumption of the engine when the auxiliary equipment is not in use, to reduce unnecessary wear of the hydraulic system, to facilitate replacement of damaged or worn units, as well as repair of parts of the hydraulic system, and to allow a vehicle provided with such a PTO to be driven even if the hydraulic system has been damaged.